This invention relates to apparatus and a method for monitoring variations in distances and in particular, but not exclusively, the clearance between relatively rotating components such as rotor blades and their casings in axial flow compressors and turbines.
In the field of gas turbine engine design and operation it is particularly desirable to monitor accurately the clearances between the tips of turbine or compressor rotor blades and their surrounding casings, because excessive clearances reduce engine efficiency, whilst contact between blades and casings causes damage.
There is a requirement for blade tip clearance monitoring apparatus and a method which enables high accuracy, real time clearance measurements to be obtained, preferably for each individual blade as it passes the monitoring position at operational speeds, under the severe operational and environmental conditions in and adjacent to gas turbine flow passages. These severe conditions include very fast blade tip speeds, vibrations, high pressures, velocities and temperatures of the turbine gases. Whilst being able to cope with these demands, the monitoring means should preferably be simple, reliable for long term operation in an engine in normal airline or military service, cheap, and easy to install, calibrate and maintain. When used in active tip clearance control systems, it should give an output signal which is representative of the clearance distance.
GB 2066449 discloses an optical device for monitoring variations in the distance between an object and a datum. Although this optical device may be designed and used for monitoring variations in distance there are inherent disadvantages associated to it particularly when used for turbine tip clearance measurements. The disadvantages are apparent when one considers the extreme environment of the turbine and the construction of the optical device. The device, disposed to the casing and radially outward of the tips of the turbine blades, is subject to a severe and changing thermal gradient which is dependent on the engine cycle. Under such conditions the refractive index of the optical elements vary leading to difficulties in calibration and thereby accuracy. Thermal expansions of the casing and optical component holders of the casing lead to further inaccuracies.
It is an object of the present invention to provide improved apparatus and an improved method for monitoring the clearances of the tips of blades to their surrounding casing.
According to the present invention a device for monitoring variations in the distance between an object and a datum, comprising: a light source, a light receiving surface, an astigmatic system for projecting an astigmatic image of the light source onto the object, the astigmatic system including the datum; and image shape detector means for detecting changes of shape of the astigmatic image on the object due to the variation in the distance between the object and the datum, and for producing a monitor signal whose value is dependant on the shape of the astigmatic image on the object: the astigmatic system being arranged such that light reflected from the astigmatic image on the object passes back through the astigmatic system and is thereby projected onto a light receiving surface of the image shape detection means as an astigmatic image of the astigmatic image on the object; wherein the astigmatic system comprises a zone plate.
Preferably, the astigmatic system comprises a first zone plate and a third zone plate and an astigmatic lens. It is further preferable that the astigmatic system comprises a first zone plate and a third zone plate and an astigmatic zone plate.
Preferably, the light receiving surface of the image shape detector means is located in the proximity of the light source. Moreover it is preferable, that the light receiving surface of the image shape detector means is disposed symmetrically around the source.
Preferably, the light receiving surface of the image shape detector means comprises a plurality of light receiving locations, the plurality of light receiving locations are disposed symmetrically around the light source. Alternatively, a beam splitter means is located between the light source and the astigmatic system thereby to deflect light onto the light receiving surface of the image shape detector means.
Preferably, the image shape detector means comprises a photo-cell, the photo-cell is of the dual axis position sensor type having X and Y axis outputs which outputs are combined to produce the monitor signal, the light receiving surface of the photo-cell comprising the light receiving surface of the image shape detector means.
Preferably, the device comprises a light guide means and the image shape detector means comprises a photo-cell, the photo-cell is of the dual axis position sensor type whose X and Y axis outputs are combined to produce the monitor signal, the light receiving surface of the photo-cell being remote from the light receiving surface of the image shape detector means and connected thereto by light guide means.
Alternatively, the device comprises light guide means, and the image shape detector means comprises a plurality of photo-cells, the plurality of photo-cells outputs are combined to produce the monitor signal, the light receiving surfaces of the photo-cells comprising the light receiving surface of the image shape detector means.
Alternatively, the device comprises light guide means, and the image shape detector means comprises a plurality of photo-cells, the plurality of photo-cells"" outputs are combined to produce the monitor signal, the light receiving surface of the photo-cells being remote from the light receiving surface of the image shape detector means and connected thereto by light guide means.
Alternatively, the device comprises light guide means, and the image shape detector means comprises a plurality of photo-cells, the plurality of photo-cells"" outputs are combined to produce the monitor signal, the light receiving surface of the photo-cells being remote from the light receiving surfaces of the image shape detector means, each of the photo-cells being connected by light guide means to a corresponding one of the plurality of light receiving locations.
Alternatively, the image shape detector means comprises four photo-cells, the four photo-cells"" outputs are combined to produce the monitor signal, the light receiving surfaces of the photo-cells being equi-angularly spaced around the light source.
Alternatively, the device comprises light guide means, and the image shape detector means comprises four photo-cells, the four photo-cells"" outputs are combined to produce the monitor signal and the light receiving surfaces of the photo-cells are remote from the light receiving surface of the image shape detector means, the light receiving surface of the image shape detector means comprising four locations equi-angularly spaced around the light source and each one of the photo-cells being connected to a corresponding one of the locations by light guide means.
Alternatively, the image shape detector means comprises two photo-cells, two photo-cells"" outputs are combined to produce the monitor signal and the light receiving surfaces of the photo-cells are remote from the light receiving surface of the image shape detector means, the light receiving surface of the image shape detector means comprising four locations equi-angularly spaced around the light source and each one of the photo-cells being connected to two of the locations which are diametrically opposed to each other.
In a preferred embodiment, the light source comprises an optical fibre, a light emitting end of the optical fibre and means for injecting light into the distal end of the optical fibre and the means for injecting light into the distal end of the optical fibre comprises a light emitting diode.
In a further preferred embodiment, the image shape detector means comprises: a light receiving surface, a photo-cell means and light guide means: the photo-cell means is situated remotely from the light receiving surface of the image shape detector means; and the light guide means is for transmitting light to the photo-cell means, the light receiving means having the light receiving surface of the image shape detector means.
It is preferred that the device is provided with a composite optical component for emitting and receiving light, the composite optical component comprising a light emitting end, the light guide means and an optical fibre, the optical fibre itself comprises a light emitting end which forms the light source, the light guide means and the optical fibre being bonded together to form a unitary structure.
Preferably, the photo-cell means comprises four photo-cells and the light guide means comprises four light guides, whereby the light receiving surface of the image shape detector means comprises the light receiving ends of the four light guides, the light receiving ends of the four light guides being equi-angularly spaced around the light source.
Alternatively, the photo-cell means comprises two photo-cells and the light guide means comprises four light guides, whereby the light receiving surface of the image shape detector means comprises the light receiving ends of the four light guides, the light receiving ends of the four light guides being equi-angularly spaced around the light source and each of the photo-cells being connected to two light guides whose light receiving ends are diametrically opposed to each other.
Preferably, the light receiving ends of the light guides are configured as a quadrant of a circle centred on the light source and the light receiving ends of the light guides are circular.
Preferably, the astigmatic system comprises in series collimator zone plate for collimating the light from the light source, an astigmatic zone plate for producing the required degree of astigmatism and projecting zone plate for projecting the astigmatic image of the light source onto the object.
According to yet a further preferred embodiment of the present invention, a device for monitoring variations in the distance between an object and a datum as claimed in claim 25 wherein the distance to be controlled is the clearance between blades of a turbine or compressor rotor and the surrounding casing, the device comprising a probe mounted in the casing to project the astigmatic image of the light source onto the radially outermost portions of the blades.
Accordingly, the present invention provides a method for monitoring variations in the distance between an object and a datum using a device as claimed in claim 1, the method comprising the steps of: projecting an astigmatic image of a light source on to the object by means of an astigmatic system which includes the datum; detecting the change of shape of the astigmatic image on the object due to distance variations; and producing a monitor signal whose value is dependant on the shape of the astigmatic image on the object; wherein in order to detect changes of shape of the astigmatic image on the object, light reflected therefrom passes back through the astigmatic system and is thereby projected on to image shape detector means as an astigmatic image of the astigmatic image on the object.